(1) Field of the Invention
This invention concerns prepregs comprising a resin impregnated non-woven micro-fiber glass reinforcing material. The present invention also includes electronic products that are manufactured using the electronic substrates of this invention including, but not limited to built-up printed wiring boards wherein the layers built-up with the prepregs of this invention include lased vias.
(2) Description of the Art
Electronic substrates based on inorganic woven reinforcing material coupled with thermosetting or thermoplastic polymers are well known as are woven glass fiber reinforced epoxy-based resin systems. Such woven fiber reinforced substrates form the foundation for conventional printed circuit board substrates. Although such substrates are excellent for conventional circuitry, they can suffer from a number of shortcomings when they are used to manufacture high density electronic circuitry. For example, it can be difficult to fabricate very thin structures (below 4 mil) using with the desired degree of uniformity using woven glass because the fabrication of continuous glass (silica) fiber becomes more difficult as the fiber diameter is reduced below about 5 microns. In addition, the ability to control the Coefficient of Thermal Expansion (CTE) of electronic substrates is limited by the properties of the woven glass fibers and by the density of glass. Thus conventional electronic circuit substrate technology becomes more difficult to apply to the fabrication of high density circuitry.
There is also a growing demand for multi-layer printed circuit boards with micro-vias uniting circuit layers. One of the leading technical approaches for manufacturing micro-vias involves using a laser to ablate holes from the outside of a circuit board, through one or more layers of insulation down to another layer of circuitry. Woven glass fabric reinforced prepregs are very difficult to laser drill as the glass is slow to ablate, and the amount of glass that needs to be ablated varies widely depending on location of the via hole in relation to the glass weave.
Resin coated foil (RCC.TM.) has been used to fabricate high density built-up multilayer circuit boards with lased visas. However, RCC.TM. is both expensive, and can only be used with subtractive circuitry manufacturing methods where copper is etched to create the circuit patterns. Furthermore dielectric layer formed using RCC.TM. are unreinforced with little inherent strength, high CTE, and poor dimensional stability. It is difficult to use the resin coated copper products where one wishes to plate up the circuitry after ablating the via holes.
Organic fiber reinforcements are an option to woven glass reinforceable materials. However organic fiber reinforcing materials are expensive, and they can suffer from very high moisture absorption (aramid reinforcements).
Despite the advancements made in the quality of reinforced electronic substrates, there remains a need for improvements. Specifically, there remains a need for reinforced electronic substrates that cleanly ablate when lased to produce quality through holes or vias. There also remains a need for reinforced lasable electronic substrates with improved dimensional stability, and with CTE's that more closely match the circuit layer CTE.